1. Technical Field
The present invention relates generally to fuel control systems and, more particularly, to a method of determining ethanol thresholds for a flexible fuel control system for a motor vehicle capable of operating on more than one type of fuel.
2. Discussion
Environmental and energy independence concerns have stimulated the development of alternative transportation fuels, such as alcohol fuels, for use in automobiles. Alcohol fuels include methanol and ethanol. A flexible fueled vehicle capable of operating on gasoline, or alcohol fuel, or any mixture of the two fuels, is therefore in demand. Modifications to the engine are necessary when operating on different fuels because of the different characteristics of each fuel. For example, an engine operating on ethanol or E85 (a blend of 85% ethanol and 15% gasoline) requires approximately 1.4 times the amount of fuel relative to gasoline at stoichiometry due to a lower energy content of the ethanol.
Air/fuel ratio in internal combustion engine design is typically considered to be the ratio of mass flow rate of air to mass flow rate of fuel inducted by an internal combustion engine to achieve conversion of the fuel into completely oxidized products. The chemically correct ratio corresponding to complete oxidation of the products is called stoichiometric. If the air/fuel ratio is less than stoichiometric, an engine is said to be operating rich, i.e., too much fuel is being supplied in proportion to the amount of air to achieve perfect combustion. Likewise, if the air/fuel ratio is greater than stoichiometric, an engine is said to be operating lean, i.e., too much air is being supplied in proportion to the amount of fuel to achieve perfect combustion. Alcohol fuels have a lower air/fuel ratio than gasoline at stoichiometric, so that the engine must be compensated for in the rich direction as the percentage of alcohol in the fuel increases.
With low concentrations of ethanol in the fuel, it is not necessary to modify the vehicle operation from that of pure gasoline. Also, the Environmental Protection Agency requires on-board diagnostics (OBDII) monitors to be operational at low ethanol concentrations but not at high ethanol concentrations. Therefore, it is necessary to identify ethanol thresholds for delineating between the two (i.e., high and low ethanol) operating environments.
When the ethanol concentration of fuel is learned based on an oxygen sensor feedback system such as that disclosed in U.S. Ser. No. 08/959,797, entitled "Method of Determining the Composition of Fuel in a Flexible Fueled Vehicle" to Nankee II et al. which is hereby expressly incorporated by reference herein, the fueling is modified by the product of a long-term fuel adaptive memory value and an alcohol composition multiplier. This product should be constant for any given ethanol concentration. If the fuel's ethanol concentration is increased gradually, the increase can go undetected if a detection threshold is used that forces the alcohol composition multiplier to reach a fixed value. This gradually undetected increase is referred to as "alcohol creep".
Previously, alcohol composition thresholds were fixed values that assumed the alcohol composition multiplier, which sets engine operating parameters based on the percent alcohol content of the fuel, represented the true alcohol concentration. If the alcohol concentration creeps up and the long-term fuel adaptive memory is elevated, i.e., the multiplicative correction which accounts for vehicle-to-vehicle variation, the increase in alcohol content goes undetected and the OBD II monitors undesirably remain enabled.
Therefore, it would be desirable to provide a system for improving vehicle operation by comparing alcohol thresholds to the product of the alcohol composition multiplier and the adaptive memory, or to decrease the alcohol composition multiplier's threshold as the adaptive memory increases.